The Collaborative International Dictionary of English v.0.48:

Sagacious \Sa*ga"cious\, a. [L. sagax, sagacis, akin to sagire
   to perceive quickly or keenly, and probably to E. seek. See
   Seek, and cf. Presage.]
   1. Of quick sense perceptions; keen-scented; skilled in
      following a trail.
      [1913 Webster]

            Sagacious of his quarry from so far.  --Milton.
      [1913 Webster]

   2. Hence, of quick intellectual perceptions; of keen
      penetration and judgment; discerning and judicious;
      knowing; far-sighted; shrewd; sage; wise; as, a sagacious
      man; a sagacious remark.
      [1913 Webster]

            Instinct . . . makes them, many times, sagacious
            above our apprehension.               --Dr. H. More.
      [1913 Webster]

            Only sagacious heads light on these observations,
            and reduce them into general propositions. --Locke.
      [1913 Webster]

   Syn: See Shrewd.
        [1913 Webster] -- Sa*ga"cious*ly, adv. --
        Sa*ga"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sagacious
    adj 1: acutely insightful and wise; "much too perspicacious to
           be taken in by such a spurious argument"; "observant and
           thoughtful, he was given to asking sagacious questions";
           "a source of valuable insights and sapient advice to
           educators" [syn: perspicacious, sagacious, sapient]
    2: skillful in statecraft or management; "an astute and
       sagacious statesman"

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "sagacious":
   acute, all-knowing, anticipant, anticipatory, apperceptive,
   appercipient, apprehending, apprehensive, argute, astute, aware,
   blase, cagey, clairvoyant, clever, cogent, cognizant, cosmopolitan,
   cosmopolite, critical, discerning, discriminating, divinatory,
   experienced, far-seeing, farseeing, farsighted, forehanded,
   foreknowing, foreseeing, foresighted, forethoughted,
   forethoughtful, gnostic, heady, incisive, insightful, intelligent,
   intuitive, judicious, knowing, knowledgeable, longheaded,
   longsighted, mature, matured, mindful, not born yesterday, old,
   omniscient, penetrating, perceptive, percipient, perspicacious,
   perspicuous, piercing, practiced, precognitive, precognizant,
   prehensile, prepared, prescient, provident, providential, prudent,
   ready, ripe, ripened, sage, sapient, savvy, seasoned, sensible,
   shrewd, smart, sophic, sophisticated, trenchant, tried,
   tried and true, understanding, veteran, wise, world-wise, worldly,
   worldly-wise